Wavelength division multiplexing (WDM) transmission technique employing intensity modulation optical signals (On-Off Keying) of several tens of wavelengths at 2.5 Gbps, 10 Gbps and so on has been put to practical use in terrestrial transmission systems such as access network system, metro network system, long-distance network system and the like, and submarine transmission systems. As to WDM transmission system of 40 Gbps which will soon come to practical use, development of element techniques and devices thereof accelerates, and the transmission distance and the frequency utilization efficiency equivalent to those of 10 Gbps systems are required.
As means for realizing the WDM transmission system of 40 Gbps, actively searched and developed are modulation schemes such as Optical Duobinary, CS-RZ (Carrier Suppressed-Return to Zero), DPSK (Differential Phase Shift Keying), DQPSK (Differential Quadrature Phase-Shift Keying), etc., for example. As compared with NRZ (Non Return to Zero) modulation scheme applied to conventional systems of 10 Gbps or less, these modulation schemes are promising modulation techniques as a means for realizing the WDM transmission system of 40 Gbps because part or all of frequency utilization efficiency, optical signal to noise ratio (OSNR) resistance and nonlinearity resistance are superior.
Among them, DQPSK modulation scheme is a scheme in which light having one frequency channel is quadrature-phase-modulated to transmit simultaneously two bits per one code. This scheme needs one-half the pulse repetition frequency, that is, the code transmission rate, for the data speed (for example, 40 Gbps) to be transmitted, hence the signal spectral width becomes about a half as compared with the known intensity modulation schemes. Therefore, this scheme is superior in frequency utilization efficiency, wavelength dispersion resistance, optical device transmission characteristic, etc. For this reason, application of the phase modulation scheme represented by DPSK modulation scheme and DQPSK modulation scheme is vigorously discussed in the field of optical transmission systems.
The WDM transmission system employing the intensity modulation optical signal of 2.5 Gbps or 10 Gbps, which is widely put to practical use in various systems, can be stepped up by increasing the number of wavelengths to be multiplexed. For example, C-band optical amplifier can transmit a maximum of 40 waves when the wavelength interval is 100 GHz (about 0.8 nm) because some of the C-band optical amplifiers have a signal optical bandwidth of about 32 nm. The WDM transmission system itself is capable of transmitting 40 waves (channels), but the administrator gradually increases the number of wavelengths to be used according to operational state of the network.
On the other hand, in order to suppress FWM (Four Wave Mixing) that has been a problem in WDM transmitting systems employing known NRZ modulation scheme or the like in an optical fiber applied as an optical transmission line, SMF (Single Mode Fiber) having a relatively large dispersion amount is employed. When a long-distance transmission line is configured, there occurs a problem that an effect of SPM (Self Phase Modulation) is produced.
In recent years, it is discussed that a dispersion shifted fiber (NZDSF: Non-Zero Dispersion Shifted Fiber) or the like, which has a relatively small wavelength dispersion per unit length, is applied as the transmission fiber to have a balance of effects of SPM, FWM and the like described above, thereby to obtain the optimum reception signal quality.
In the case where an optical fiber having a relatively small wavelength dispersion per unit length such as NZDSF is applied as the optical transmission line as stated above, when the number of wavelengths to be multiplexed is increased in order to step up the system as stated above, the wavelength interval is narrowed, the quantity of walkoff between the wavelengths is decreased, and the effect of cross phase modulation (XPM), which is a nonlinear effect between the wavelengths, is increased. XPM is a phenomenon that the refractive index of the optical fiber changes in proportion to a change in intensity of an optical signal having a certain wavelength and gives phase modulation to an optical signal having another wavelength.
FIGS. 16A through 16C are conceptual diagrams illustrating a phenomenon of the cross phase modulation caused by optical pulses. In FIG. 16A, λ1 and λ2 are optical pulses having different wavelengths. Here, it is assumed that speed of the optical pulse λ1 is faster than that of the optical pulse λ2 because of the dispersion characteristic of the optical fiber in which the optical pulses λ1 and λ2 are propagated. When the two optical pulses are propagated in the optical fiber, leading edge of the optical pulse λ1 begins overlapping on trailing edge of the optical pulse λ2 as illustrated in FIG. 16B because the optical pulse λ1 travels faster than the optical pulse λ2.
On this occasion, the leading edge of the optical pulse λ2 is affected by phase shift due to red chirp induced by the leading edge of the optical pulse λ1, which causes the phase of the optical pulse λ2 to be delayed. Further, when transmission of the optical pulses λ1 and λ2 progresses, the optical pulse λ1 outstrips the optical pulse λ2 and the trailing edge of the optical pulse λ1 overlaps on the leading edge of the optical pulse λ2, as illustrated in FIG. 16C. On this occasion, the leading edge of the optical pulse λ2 is affected by phase shift due to blue chirp induced by the trailing edge of the optical pulse λ1, which causes the phase of the optical pulse λ2 to lead.
Meanwhile, as relating techniques, there are non-patent document 1 and patent document 1 below:    Non-patent Document 1: G. Charlet et. al., “Nonlinear Interactions Between 10 Gbps NRZ Channels and 40 Gb/s Channels with RZ-DQPSK or PSBT Format, over Low-Dispersion Fiber”, Mo3.2.6, ECOC2006;    Patent Document 1: Japanese Patent Application Laid-Open Publication No. H08-125605
A market demand is to provide a wavelength division multiplexing transmission system 100 which transmits a phase modulation signal of 40 Gbps in RZ-DQPSK modulation scheme or the like and intensity modulation signals of 10 Gbps (or 2.5 Gbps) in NRZ modulation scheme or the like in a mixed form, as illustrated in FIG. 17. In this case, it is supposed that an existing transmission system is stepped up to a transmission system in which a phase modulation optical signal is arranged in channel arrangement which has been usable for wavelength division multiplexing of intensity modulation optical signals.
Namely, in the optical transmission system 100 in which a plurality of OADM (Optical Add Drop Multiplexing) nodes 101 are connected in multiple stages via NZDSFs 102, optical amplifiers 103 and DCFs (Dispersion Compensating Fibers) 104 as a transmission line as illustrated in FIG. 17, it is supposed to step up a transponder configuring each transmission channel of OADM nodes 101-1 and 101-2 so that the transponder can transmit and receive the DQPSK optical signal of 40 Gbps.
In the optical transmission system 100 illustrated in FIG. 17, an add port of the OADM node 101-1 accommodates four transponders (TRPNs) 106 outputting respective NRZ intensity modulation optical signals having wavelengths of λn−2, λn−1, λn+1, and λn+2 via a multiplexer 107, along with a transponder 105-1 outputting a phase modulation optical signal in channel #n having a wavelength λn (n being an integer equal to three or more) in DQPSK modulation scheme, via the same.
Namely, the transponder 105-1 outputs a DQPSK optical signal of 40 Gbps differently from the transponders 106 of other channels outputting NRZ optical signals of 10 Gbps, while a transponder 105-2 receiving a wavelength λn in a channel #n outputted through a drop port of the OADM node 101-2 receives the DQPSK optical signal of 40 Gbps. Incidentally, reference character 108 designates a demultiplexer which separates the optical signal of λn from optical signals having other wavelengths dropped by an OADM node 101-2.
On this occasion, the RZ-DQPSK signal of 40 Gbps is optical-phase-shifted due to the above-mentioned XPM caused by an NRZ signal of 10 Gbps (2.5 Gbps) and waveform thereof is noticeably deteriorated, which reversely affects in long-distance transmission, as illustrated in a result of transmission simulation in FIG. 18 and the above non-patent document 1. Particularly, such deterioration of the waveform is more noticeable in NZDSF and DSF (Dispersion Shifted Fiber), which are fibers having small transmission line dispersion coefficient than a fiber (SMF) having large transmission line dispersion coefficient.
Namely, since the quantity of walkoff between the wavelengths is relatively large when an optical fiber having a sufficiently large transmission line dispersion coefficient is applied as the optical transmission line, the optical signal (unit section from the leading edge to the trailing edge) having a wavelength λ1 illustrated in FIGS. 16A through 16C mentioned above can catch up with and outrun the optical signal having a wavelength λ2 within one span. Accordingly, the amount of phase shift due to the generated red chirp and blue chirp illustrated in FIGS. 16A through 16C mentioned above is cancelled, hence the effect of XPM on the transmission characteristic is relatively small.
However, when an optical fiber having a relatively small transmission line dispersion coefficient is applied as the transmission line, the optical signal (unit section from the leading edge to the trailing edge) having the wavelength λ1 illustrated in FIGS. 16A through 16C mentioned above cannot secure a sufficient amount of walkoff between the wavelengths to catch up with and outrun the optical signal having the wavelength λ2 within one span. As a result, the amount of phase shift caused by the red chirp and blue chirp generated in FIGS. 16A through 16C is not cancelled, hence the effect of XPM on the transmission characteristic is relatively large.
Particularly, when the optical signal having a residual phase shift amount is a phase modulation optical signal, the residual phase shift amount becomes direct noise components of data symbol, which degrades the transmission performance. In FIG. 18, when the phase modulation optical signal (RZ-DQPSK signal) of 40 Gbps is of a wavelength division multiplexed optical signal in which one or five waves are multiplexed (see A and B in FIG. 18), the value of Q penalty of the reception signal remains in excellent reception signal quality even with an increase in input power to an optical fiber, which is the transmission line. However, when a mixture of four intensity modulation optical signals (NRZ signals) of 10 Gbps and the phase modulation optical signal of 40 Gbps are transmitted, the value of Q penalty of the reception signal illustrates deterioration in reception signal quality with an increase in input power to the optical fiber. Particularly, when polarized wave of the phase modulation optical signal is parallel to polarized waves of the other four intensity modulation optical signals (see D in FIG. 18), it can be said that the degradation in reception signal quality is noticeable as compared with the case where the polarized wave of the phase modulation signal is orthogonal to the polarized waves of the other four intensity modulation optical signals (see C in FIG. 18).
The non-patent document 1 mentioned above illustrates a result that when an intensity modulation optical signal of 10 Gbps is mixed with a phase modulation optical signal of 43 Gbps and transmitted in NZDSF, which is a fiber having a small wavelength dispersion value, the reception signal quality is deteriorated when the intensity modulation optical signal of 10 Gbps is positioned adjacent to the wavelength of the phase modulation optical signal of 43 Gbps, as compared with an arrangement in which the phase modulation optical signal of 43 Gbps is positioned in each wavelength channel. Incidentally, the non-patent document 1 illustrates a result that Q value illustrating the reception signal quality is more deteriorated than WDM transmission of only phase modulation optical signals of 43 Gbps even when the state of polarization is in the optimum (orthogonal) state, and the Q value is deteriorated about 3 dB in the polarization parallel state.
Generally, dispersion compensation is performed in order to suppress waveform deterioration in a repeating apparatus in an optical transmission system. When a difference in propagation delay time of an optical signal at an adjacent wavelength is compensated by this dispersion compensating function in each repeating stage, the bit arrangement on the time axis is recovered in each repeating stage, and residue of the above-mentioned red chirp is accumulated.
In other words, when an intensity modulation optical signal and phase modulation optical signals are transmitted in a mixed form as stated above, it is necessary to take more aggressive measures to suppress the effect of XPM than the case of known WDM transmission of only intensity modulation optical signals of 10 Gbps or WDM transmission of phase modulation optical signals when the transmission system is stepped up.
The non-patent document 1 does not provide a means for suppressing the effect of XPM on a phase modulation optical signal when an intensity modulation optical signal and the phase modulation optical signals are transmitted in a mixed form as stated above. Patent document 1 relates to an optical communication system which reduces deterioration of an optical signal wave due to self phase modulation effect, not providing a technique for suppressing deterioration of the transmission quality due to XPM in a network in which an intensity modulation optical signal and a phase modulation optical signal exist in a mixed form.